If you have a valid product key, select Change product key, and then enter the 25-character product key. If you don’t have a valid product key, select Go to Microsoft Store, and follow the instructions to buy a license for Windows.

Note: Your product key should be in the confirmation email you received after buying Windows, included with the packaging that came with your PC, or on the Certificate of Authenticity (COA) attached to the back or bottom of your PC.